The present invention relates to a decurl device which detects a curl of a sheet and uncurls the sheet, and an image forming apparatus provided with the decurl device.
An electrophotographic image forming apparatus, i.e., a copy machine, a printer, a facsimile or a multi-functional machine having the functions thereof, performs image formation by transferring a toner image, obtained by developing an electrostatic latent image formed on an image carrier, with a developing device, on a sheet directly or via an intermediate transfer member, then feeding the sheet through a pressuring position between a heat roller for fixing or a fixing belt and a pressure roller to fix the image.
In a fixing process, dehydration of a sheet causes so-called curling, a phenomenon such that the sheet after fixing warps upward or downward in a wavy form. Particularly, such curling of a sheet tends to noticeably occur due to increased amounts of toners supplied to the sheet at the time of forming a color image, the use of multifarious kinds of sheets, etc.
A curl of a sheet raises a problem particularly in a case of forming images on both sides of a sheet or a case of making an article bound by bookbinding or so in a finisher.
There is a case where a decurl member for correcting a curl of a sheet which occurs after fixing is used to avoid reduction in image quality or so originating from paper jamming, reduced sheet storability in a sheet ejecting section, a transfer failure at the time of double-side image formation, or the like all caused by such a curl.
A sheet curl control apparatus described in Unexamined Japanese Patent Application No. 05-238624 (FIG. 4) has a decurler and adjusting means which adjusts the decurling action of the decurler, detects the amount of curling by measuring a time interval between the first interruption of an infrared ray irradiated from one light source by passing of the leading edge of a sheet and the next interruption of the infrared ray using infrared sensors at two locations, and changes the decurling action according to the curling amount.
In above-mentioned sheet curl control apparatus, as the sensors are disposed downstream of a sheet in the sheet conveying direction, the sensors detect a curl at the leading edge of a sheet in the sheet conveying direction but cannot detect a curl which occurs on the entire surface of the sheet. Because a curl is detected by a horizontal sheet conveying section, the curling amount changes due to the dead weight of the sheet, disabling detection of the accurate curling amount.
A sheet conveying device described in Unexamined Japanese Patent Application No. 2002-60112 (FIG. 1) detects a curling amount with a reflection type displacement sensor, and performs such control as to change the sheet suction force generated by a suction fan according to the detected sheet curling amount.
Because the sheet conveying device also has the sensor disposed at an end portion of a sheet to detect the amount of displacement of the leading edge of the sheet in the sheet conveying direction at the horizontal sheet conveying section, it also cannot detect an accurate curling amount.
A decurl system described in Unexamined Japanese Patent Application No. 2002-80158 comprises decurl devices, curling amount measuring devices and correction amount modifying means, and detects a curling amount by the displacement of an actuator or a reflection type displacement sensor.
In this system, a curl is detected in the vicinity of the decurl device on the downstream side in the sheet conveying direction to detect the curling amount after the sheet is uncurled.